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Ultrasonic sensor with stainless steel housing

Reliable solution for all kinds of level measurement
Ultrasonic sensor with stainless steel housing

Ultrasonic sensors are frequently used to monitor the levels of liquids, granules or pastes. However, many of these applications involve media whose liquids or emitted gases can have a long-term detrimental effect on the materials and therefore also the functioning of standard sensors. These aspects were specifically taken into account in the design of the Unar ultrasonic sensor range.

Rudolf Zingg

Ultrasonic sensors for level measurement are based on the principle of propagation time measurement. They employ the surrounding air to convey the sound energy, in which sound is propagated at a velocity of around 330 m/s under normal conditions. The time between emission and reception is measured using a high-precision electronic timer. This timer permits accurate distance measurements, a constant resolution in the region of a few tenths of a millimetre and precise, reproducible switching points. The transducer is designed as both an emitter and a receiver and is optimized for frequencies far beyond the range of human hearing. When excited by an oscillator, the diaphragm of the transducer emits short ultrasonic packages, referred to as bursts, in the surrounding air, which carries and propagates the sound waves. Each emission sequence is followed by a period of reception. In operation, the transducer and the electronic circuit alternate constantly between emission and reception. When the emitted sound packages strike an object, such as the surface of a liquid, the sound reflects from this surface and a part of the energy is returned to the transducer or the sensor. If the latter registers a defined number of successive signals within a time window programmed by the user, the sensor output responds. As mentioned above, the surface properties and the colour of the object to be detected are irrelevant with this method of measurement. As long as an echo of the emitted sound energy is returned to the receiver with a sufficient amplitude, it is fundamentally possible to measure the distance from the reflecting body. For this reason, ultrasonic sensors are mainly used to detect and scan objects which are otherwise difficult to sense.
In industrial processes, this generally means measurements on the surfaces of bulk materials, liquids, pastes and similar substances. The vapours and gases emitted by these substances, which can be very aggressive, have become more of a problem than ever for the majority of sensors in use today because they can corrode and attack the sensor front. This can lead to permanent damage or even total failure.
With Parylene coating
The 1.4435 stainless steel grade employed for the housing makes a substantial contribution to protecting the sensor. This material, which is in widespread use in the food and process industries, resists all aggressive effects. However, the far greater challenge is to protect the transducer of the sensor, which is usually particularly exposed to the aggressive media, as effectively as possible against a variety of corrosive substances. Of course, it must do this without any negative effects on the properties of the transducer diaphragm and hence the emission and reception characteristics of the transducer, with its typically narrow sonic beam angle. This very narrow sonic beam is one of the properties of the ultrasonic transducers used in Unar 18 sensors. It permits simple sensing in containers with small diameters or detection through narrow openings – something that is frequently demanded in process and medical engineering. The ideal solution to comply with all such specifications is Parylene condensation coating. Parylene combines the specified properties of good adhesion to the materials to be protected – ceramic and PUR – with an excellent barrier effect in relation to the majority of substances used in modern processes, such as inorganic acids and alkalis, organic solvents and water vapour, even if a layer thickness of just a few microns is stipulated. At first sight, there are no noticeable mechanical differences between a Unar and a standard ultrasonic sensor of identical size. In contrast to other solutions, it was possible to avoid a bulky mechanical covering of the active surface thanks to the Parylene coating. The M18 thread remains unchanged over the entire length of the housing. This especially simplifies installation.
Deflecting sound energy by 90°
Most level-measuring ultrasonic sensors are mounted vertically with the active surface facing downwards, as they are nor-mally adjusted from above to the media to be monitored. However, if this is impossible due to a lack of space, a screw-on 90° deflector made of stainless steel can be supplied for the Unar sensor series. Both switching and distance measuring sensors can be operated with this accessory. Its influence on the measuring accuracy of the level sensor is negligible. Assuming it is correctly fitted, the deflector even provides additional mechanical protection for the Parylene-coated transducer. Owing to the deflection angle, the Unar is equally unproblematic if the installation depth is very limited.
Ultrasonic level sensors can optimally be integrated in the customer’s application, not only from a mechanical viewpoint but also electrically. All level sensors in the Unar series are equipped with a teach-in function that allows the measuring range to be programmed explicitly for each application up to a maximum full-scale value of 1000 mm. The effective direction of the analogue output signal – rising or falling as a function of distance – can be changed at any time. Programming is carried out either directly on the sensor with a pushbutton or externally via an electrical teach-in input. Access is electronically locked for a few minutes after programming, signi- ficantly increasing reliability. Baumer delivers a solution with an integrated liquid crystal display for all applications where the level has to be indicated directly at the place of measurement. The measured data visible on this display is also accessible as a 4…20 mA current signal.
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Baumer Products
Automation 2008
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